1. Field of the Invention
This invention relates generally to downhole tools used for boring petrolitic wells and specifically to a downhole drilling-fluid-powered motor with integrated rotary torque-intensifying percussion tool useful for driving diamond drag-type drill bits.
2. Background Art
In normal drilling conditions polycrystalline diamond compact (PDC) drill bits can bore through certain earthen formations with amazing speed and efficiency. However, when drilling with aggressive PDC bits, an undesirable effect known as “stick-slip” can occur when the drill bit encounters a particularly hard formation, such as an inter-bedded stringer. In stick-slip, the drill bit catches or seizes in the subsurface formation when there is insufficient torque at the bit to shear the rock. Despite the momentarily slowing or stopping of the drill bit, the drill string continues to rotate and torsional potential energy accumulates in the length of the upper end of the drill string. Once sufficient torque is generated at the bit to shear the rock, or to disengage the bit from the formation due to shortening of the drill string as it winds up, the bit violently breaks free at higher speeds than normal. The release of wound-up torsional energy as the bit spins may cause bit chatter or repeated impacting of the diamond cutters against the rock face. PDC cutters perform well under constant loading but are subject to failure under impact loading. Because of this characteristic, the stick-slip phenomenon tends to compromise the integrity of the diamond inserts and may cause cutter damage, bit failure and concomitant loss of drilling time.
One methodology in dealing with stick-slip is centered on preserving the bit. To achieve this objective, the bit is made more passive and heavy-set, and/or weight-on-bit is reduced. However, these changes make drilling less efficient and result in lower rates of penetration. Alternatively, roller cone bits are employed. But, roller cone bits are less efficient, and they also increase the risk of lost components in the well bore.
A more effective solution is to employ a relatively high-frequency rotational impact to the drill bit in conjunction with steady drill string torque. The repeated percussive impacts applied to the drill bit result in periodic torque impulses to promote shearing of the rock formation. Percussive impacts are provided by a rotational impact tool disposed in the drill string above the drill bit. Such a tool has been shown to be effective in reducing bit lock-up and minimizing harmful drill string windup. One such device is disclosed in U.S. Pat. No. 6,742,609 entitled “Rotational Impact Drill Assembly” and issued on Jun. 1, 2004 to inventors Peter J Gillis, Ian G. Gillis, and Craig. J. Knull, which is incorporated herein by reference.
The impact-generating tool mentioned above may be included in a bottom hole assembly between a downhole mud motor and a diamond drill bit. The downhole motor converts hydraulic energy of the drilling fluid, or mud, into mechanical energy in the form of torque and rotational speed for rotating a drill bit via the impact generator.
It is also generally desirable to shorten the length of the bottom hole assembly (i.e., the “bit-to-bend” length) to reduce the radius in a transition from a vertical to a horizontal bore. Accordingly, it is desirable to gain the advantages of impact generation but without suffering the penalty of an increased bit-to-bend length due to the inclusion of an impact generating tool in the bottom hole assembly. Such would control torque and minimize erratic tool face, which has plagued fixed cutter bits in tight turns, thereby enhancing the ability to drill high-build-rate sections with a PDC bit.
3. Identification of Objects of the Invention
A primary object of the invention is to provide an apparatus characterized by a short bit-to-bend length that facilitates the ability to drill high-build-rate well section with a drag-type drill bit.
Another object of the invention is to provide an apparatus that improves the ability to drill build sections with a drag-type drill bit and continue to drill horizontal sections without the need to trip the drill string out of the hole.
Another object of the invention is to provide an “all-in-one” apparatus that minimizes torque build-up, improves the life of drag-type bits, and that is capable of drilling directional wells.